The subjects under investigation are: 1. The hydrolysis of misacylated-tRNA's by aminoacyl-tRNA synthetases. 2. The accuracy of the overall translation process and the factors which affect it. 3. The mechanism of the amber suppressor Su7 ion, particularly as regards its phenotypic change in vivo from TRP-tRNA to GLN-tRNA. 4. The mechanism of biosynthesis of isoleucyl-tRNA synthetase. 5. The control of tRNA isoacceptor levels. 6. The existence of DNA binding proteins which require aminoacyl-tRNA. 7. The existence and nature of labile features of the sequence of tRNA Ile (E. coli) which distinguish the native molecule from the altered form usually studied in vitro.